Single bi-temperature thermal storage tank for application in solar thermal plant

ABSTRACT

Thermocline storage tanks for solar power systems are disclosed. A thermocline region is provided between hot and cold storage regions of a fluid within the storage tank cavity. One example storage tank includes spaced apart baffles fixed relative to the tank and arranged within the thermocline region to substantially physically separate the cavity into hot and cold storage regions. In another example, a flexible baffle separated the hot and cold storage regions and deflects as the thermocline region shifts to accommodate changing hot and cold volumes. In yet another example, a controller is configured to move a baffle within the thermocline region in response to flow rates from hot and cold pumps, which are used to pump the fluid.

This application claims priority to U.S. Provisional Application No.61/183,042, filed on Jun. 1, 2009.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The government may have certain rights to this invention pursuant toContract No. DE-FOA-0000065 awarded by the United States Department ofEnergy.

BACKGROUND

This disclosure relates to a thermocline storage tank suitable for usein a solar power system, for example.

Typically two large storage tanks are used for molten salt thermalstorage solar plants (power tower or trough designs). These tankstypically can hold well over 50 million pounds of salt each and measureover 125 ft (38 m) in diameter and over 40 ft (12 m) tall. In a powertower system the cold tank is typically fabricated out of a carbon steelmaterial and stores salt at about 550° F. (288° C.). The hot tank cantypically be fabricated out of a stainless steel or other high strengthalloy and stores salt at about 1050° F. (566° C.).

Each tank is sized to store the entire plant working inventory of moltensalt. In the morning, cold salt at 550° F. (288° C.) is pumped out ofthe cold tank into the solar energy receiver where it is heated to 1050°F. (566° C.) and then stored in the hot tank. When required to producesteam and electrical power, hot salt is pumped out of the hot tank andsent to the steam generator system where it is cooled back to 550° F.(288° C.) and returned to the cold thermal storage tank. In thisfashion, salt is “shuttled” back and forth between the two tanksfollowing a diurnal cycle. Thus, there is twice the storage capacity inthe combined volume of the two tanks as there is molten salt. At timesone tank is generally full and the other generally empty and other timesboth tanks are partially full.

Salt tanks for large solar power plants are quite expensive and includeelectrical heat tracing or other forms of heaters, thermal insulation,cooled foundation, instrumentation, and other supplementary equipmentincluding a support structure. To date, designers have studied replacingthe hot and cold tanks with a single tank but have not solved theproblem of effectively and efficiently precluded the mixing of the hotand cold fluids in a single tank.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure can be best understood fromthe following specification and one or more drawings, the following ofwhich is a brief description.

FIG. 1A is a general schematic view of a solar power tower system.

FIG. 1B is a graph generally depicting hot and cold regions of a fluidseparated by a thermocline region.

FIG. 2A is a schematic view of an example thermocline storage tank for asolar power system.

FIG. 2B is a top elevational view of a baffle shown in FIG. 2A.

FIG. 2C is a partial top elevational view of spaced apart baffles shownin FIG. 2A.

FIG. 3A is a schematic view of another example thermocline storage tankfor a solar power system.

FIG. 3B is a top elevational view of a baffle shown in FIG. 3A.

FIG. 3C is a partial cross-sectional view of the baffle shown in FIG. 3Btaken along line 3C-3C.

FIG. 4A is a schematic view of yet another example thermocline storagetank for a solar power system.

FIG. 4B is a schematic of a control system used to control a movablebaffle depicted in FIG. 4A.

DETAILED DESCRIPTION

Referring to FIG. 1A, a solar power tower system 20 includes a highconcentration central receiver system 22 having a reflector assembly 24coupled to a tower structure 25 at a predetermined height above groundto receive solar radiation S. Thousands of sun-tracking minors orheliostats 26 reflect solar radiation S onto the reflector assembly 24.

Molten salt or other thermal transfer fluid is communicated to and froma single bi-temperature thermal storage tank or thermocline storage tank27. The thermocline storage tank 27 includes a cold storage region 28and a hot storage region 30 in a common tank. “Cold” salt, which isaround 550° F. (288° C.) in one example, is communicated from the coldstorage region 28 through the central receiver system 22 where it isheated. The “hot” thermal transfer fluid, in the example, salt at around1050° F. (566° C.), is then communicated to the hot storage region 30.When power is required, the hot molten salt is pumped to a steamgenerator system 32 that produces steam. The steam drives a steamturbine/generator 34 that creates electricity for communication to apower grid 38. The salt is returned to the cold storage tank system 27(from the steam generator system 32), where it is stored and eventuallyreheated in the central receiver system 22. It should be understood thatalthough a particular component arrangement is disclosed in theillustrated embodiment, any arrangement that utilizes a singlebi-temperature thermal storage tank would also benefit from thedisclosed examples.

A thermocline storage tank is based on the principle that hot fluid in aquiescent environment tends to rise and stay above colder fluid in thesame tank. This phenomenon is also known as thermal stratification,which is graphically illustrated in FIG. 1B as temperature versusdistance from the bottom of the storage tank. A thermocline orthermogradient region is the layer in which temperature changes morerapidly with change in depth than do the temperatures in the layersabove or below. As the temperature of the hot fluid decreases in the hotstorage region 30, its density increases and it tends to “fall” unlessthe fluid below it is also cooling at an equal or greater rate. Therelative densities of the fluid determine its elevational positionwithin the tank. The cold fluid in the cold storage region 28 is muchmore dense than the hot fluid in the hot storage region 30. The factthat molten salt has a relatively low thermal conductivity tends to aidin this stratification effect. Molten salt can be thought of as liquidinsulation. A thermocline region 40 naturally develops and separates thecold and hot storage regions 28, 30. However, this natural separationdoes not provide efficient enough storage for use in solar powersystems.

FIGS. 2A-2C shows a thermocline storage tank having a wall 41operatively secured to a bottom wall 42 to provide a cavity 45. The wall41 includes a portion 43 having a geometry or shape interiorly arrangedwithin the storage tank. A fixed baffle assembly 46 is arranged in thethermocline region. A baffle is an obstruction used to substantiallyseparate the cold and hot storage regions 28, 30. The baffle assembly 46includes vertically spaced first, second and third baffles 50, 52, 54each providing a perimeter 56. A skirt 44, which is a border of thebaffle assembly 46, is provided on the perimeter 56 and has a geometrybased upon the portion 43, for example, of a complementary shape to theportion 43 such that the portion 43 and skirt 44 are in a very closefitting relationship. In one example, the skirt 44 is provided by theportion 43 such that the skirt 44 and portion 43 are integrated with oneanother. The number of baffles is exemplary. The first, second and thirdbaffles 50, 52, 54 are spaced over greater than 5% of the verticalheight of the tank to ensure that the shifting thermocline regionremains within the baffle assembly 46 during operation. The perimeters56 are arranged in close proximity to and fixed relative to the outerwall 44. Support posts 48 extend vertically from the bottom wall 42inboard of the outer wall 44 and operatively connect to at least onebaffle. In the example, the support posts 48 extend through apertures 49in the second and third baffles 52, 54 to the first baffle 50. Brackets60 may be used to support the first, second and third baffles 50, 52, 54relative to one another.

The flows in and out of the hot and cold pools disturb the inherentquiescent nature of the thermal stratification. The baffle assembly 46limits mixing of fluid between the cold and hot storage regions 28, 30by substantially physically separating the cavity 45 into the cold andhot storage regions 28, 30 as well as reducing thermal conductivitybetween the fluids. The baffle assembly 46 may include openings orperforations 58 in the baffles, as illustrated by the first baffle 50 inFIG. 2B. Perforations 58 between first and second baffles 50, 52, forexample, may be indexed relative to one another to obstruct the freeflow of fluid between the cold and hot storage regions 28, 30 duringpumping. The baffle assembly 46 has a limited number of flow paths torestrict vertical flow and energy transfer. The combined volumes of thehot and cold storage regions 30, 28 are equal to the total saltinventory and not twice the inventory as required by the current twotank designs. The lower portion of the single tank is exposed to onlycold salt and therefore is much less costly to design and support.

In one example, one or more immersion heaters 62 may used to heat thehot storage region fluid, and trace heaters 64 may be used to heat thecold storage region fluid. Cold salt is pumped from one or more coldsalt pumps 68 that withdraw fluid from the cold storage region 38through a cold salt supply line 74. The cold salt supply line 74 extendsthough apertures 75 in the baffle assembly 46. The salt is heated in thereceiver system 22 to 1050° F. (566° C.) and re-enters the tank througha hot salt return line 72, discharging to the hot storage region 30.Salt is withdrawn from the hot storage region 30 at 1050° F. (566° C.)using one or more hot salt pumps 66 through a hot salt supply line 70and flows to the steam generator 32 before returning through the coldsalt return line 76. The cold salt return line 76 extends throughapertures 79 in the baffle assembly 46 to the cold storage region 30.The cold salt return line 76 is insulated using insulation 77 as itpasses through the hot upper pool to minimize parasitic heat exchange.

FIGS. 3A-3C show a thermocline storage tank with a thin thermoclineregion surrounding a thin-vertically movable, flexible horizontal baffle146 that adjusts with the volume of cold and hot salt. The tank ingressand egress flows are the same as described above relative to FIG. 2A.The baffle 146 is considerably thinner than the baffle assembly 46 sinceit does not have to contain the constantly changing hot/cold poolinterface. The baffle 146 acts as a diaphragm and may be provided by anexpandable steel membrane, with, for example, corrugations 78, as shownin FIGS. 3B and 3C. The baffle 146 is configured to expand vertically ineither direction to accommodate about 95% of either the hot and coldcavities. The baffle 146 expands into the hot and cold pools as hot andor cold salt is added/removed, varying the hot and cold volumes andshifting the thermocline region.

The baffle 146 includes a perimeter 156 that that may be substantiallysealed and affixed relative to the wall 41 to separate the cavity 45into cold and hot cavities, respectively providing the cold and hotstorage regions 28, 30. In the example, the portion 43 of the wall 41 isintegrated with and provides the skirt 44, such that the portion 43provides at least a majority of the vertical portion of the wall 41. Inone example, the aperture 75 provides a flow area between the cold saltsupply line 74 and the flexible baffle 146 is configured to permitlimited fluid flow between the hot and cold cavities. The flow areagenerally corresponds to less than 1% of a horizontal cross-sectionthrough the tank cavity at the thermocline region. Alternatively, thecold pumps can be relocated within a small cold salt region along oneside of the wall 41.

Another example storage tank is shown in FIGS. 4A and 4B. A winch system84 is shown that can vertically drive a movable baffle 246 as therelative volume of hot and cold salt in the tank changes, thusincreasing the hot pool capacity with a corresponding decrease in coldpool capacity, and vice versa. The winch system 84 includes a driveelement 90 driving a drum 88 about which a cable 86 is wrapped. Thecable 86 is connected to the baffle 246. Two winch systems 84 are shown,although fewer or more may be used.

One or more pumps 66 (hot supply), 68 (cold supply), 92 (hot return), 94(cold return) are used to pump fluid into and out of the cold and hotstorage regions 28, 30, which shifts the thermocline region. Thelocation of the thermocline region can be determined by a controller 96,which may communicate with the pumps 66, 68, 92, 94 to determine theirrespective flow rates. The controller 96 is configured or programmed toadjust the vertical position of the baffle 246 based upon at least onepump flow rate to maintain the baffle 246 in the thermocline region.

Although example embodiments have been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

1. A storage tank for a solar power system comprising: a structureincluding an outer wall adjoining a bottom wall and providing a cavityconfigured to store a fluid; and multiple baffles arranged in the cavityand spaced from the bottom wall in a thermocline region, the bafflesincluding a perimeter arranged in close proximity to and fixed relativeto the outer wall, the baffles configured to substantially physicallyseparate the cavity into hot and cold storage regions.
 2. The storagetank according to claim 1, wherein the baffles are spaced verticallyfrom one another within the thermocline region.
 3. The storage tankaccording to claim 2, wherein the baffles include multiple perforationsarranged inboard of the perimeter and configured to permit substantiallylimited flow between the hot and cold storage regions through thebaffles.
 4. The storage tank according to claim 3, wherein a first setof perforations in a first baffle are offset from a second set ofperforations on a second baffle.
 5. The storage tank according to claim2, wherein the cavity includes a tank height, the baffles arrangedvertically within the thermocline region over a distance greater than 5%of the tank height.
 6. The storage tank according to claim 1, comprisingat least one generally vertical support post arranged inboard of theouter wall operatively connected to at least one baffle.
 7. A storagetank for a solar power system comprising: a structure including an outerwall adjoining a bottom wall and providing a tank cavity configured tostore a fluid; and a flexible baffle arranged in the cavity and spacedfrom the bottom wall, the flexible baffle including a perimetersubstantially sealed and affixed to the outer wall and configured tosubstantially physically separate the cavity into hot and cold cavitiesat a thermocline region, the flexible baffle configured to deflectwithin the thermocline region in response to varying hot and coldvolumes respectively provided by the hot and cold cavities as thethermocline region shifts.
 8. The storage tank according to claim 7,wherein the flexible baffle includes corrugations configured to deflectpermitting a change in hot and cold volumes.
 9. The storage tankaccording to claim 7, wherein the flexible baffle includes at least oneaperture and comprising an element extending between the hot and coldcavities through the aperture, the aperture providing a flow areabetween the element and the flexible baffle configured to permit fluidflow between the hot and cold cavities, the flow area corresponding toless than 1% of a horizontal cross-section through the tank cavity atthe thermocline region.
 10. The storage tank according to claim 9,wherein the element is a pumping conduit in fluid communication with thecold cavity.
 11. The storage tank according to claim 7, comprising hotand cold pumps respectively in fluid communication with the hot and coldcavities, the flexible membrane configured to expand in the verticaldirection to accommodate about 95% of either of the hot and coldcavities in response to pumping fluid thereto and therefrom.
 12. Astorage tank for a solar power system comprising: a structure includingan outer wall adjoining a bottom wall and providing a cavity configuredto store a fluid; a movable baffle arranged in the cavity and spacedfrom the bottom wall in a thermocline region, the baffle including aperimeter arranged in close proximity to the outer wall and configuredto substantially physically separate the cavity into hot and coldstorage regions; hot and cold pumps respectively in fluid communicationwith the hot and cold regions configured to pump fluid at flow rates toand from the hot and cold storage regions thereby shifting thethermocline region; and a controller in communication with at least oneof the hot and cold pumps and configured to move the baffle within theshifting thermocline region based upon at least one of the flow rates.13. The storage tank according to claim 12, wherein the flexible baffleincludes at least one aperture and comprising an element extendingbetween the hot and cold storage regions through the aperture.
 14. Thestorage tank according to claim 13, wherein the element is pumpingconduit in fluid communication with at least one of the pumps.
 15. Thestorage tank according to claim 13, wherein the element is a generallyvertical support post arranged inboard of the outer wall.
 16. A tankcomprising: a wall having a portion; a baffle located in a thermalgradient region; and a skirt attached to the baffle, the skirt having ageometry based at least in part on the portion, the skirt proximate tothe portion.
 17. The tank of claim 16, wherein the portion defines atleast a majority of a vertical portion of the wall.
 18. The tank ofclaim 16, wherein the baffle substantially separates a first volume andsecond volume.
 19. The tank of claim 18, wherein a hot fluid is in thefirst volume and a cold fluid is in the second volume.
 20. The tank ofclaim 19, wherein a mixing of the hot fluid and the cold fluid islimited by the baffle.
 21. The tank of claim 20, wherein the baffleincludes plural openings.
 22. The tank of claim 21, wherein the pluralopenings permit substantially limited flow between the first volume andthe second volume.
 23. A tank comprising: a wall having a portion; amembrane located in a thermal gradient region; and a skirt attached tothe membrane, the skirt having a geometry based at least in part on theportion, the skirt proximate to the portion.
 24. The tank of claim 23,wherein the portion is a vertical portion.
 25. The tank of claim 23,wherein the membrane substantially separates a first volume and secondvolume.
 26. The tank of claim 25, wherein a hot fluid is in the firstvolume and a cold fluid is in the second volume.
 27. The tank of claim26, wherein a mixing of the hot fluid and the cold fluid is limited bythe membrane.
 28. The tank of claim 27, wherein the membrane includesplural openings.
 29. The tank of claim 28, wherein the plural openingspermit substantially limited flow between the first volume and thesecond volume.